1. Field of the Invention
The present invention relates to a surface inspection apparatus, and more specifically, to an improvement in a surface inspection apparatus which obtains physical properties such as a thickness and so forth, of a predetermined film coating an inspected surface.
2. Related Art Statement
Heretofore, there has been widely known an ellipsometer as a surface inspection apparatus which obtains a thickness and other characteristics of optical properties (a refractive index, etc.) of a coating film formed on an inspected surface.
The ellipsometer projects a light beam emitted from a light source onto a predetermined area of the inspected surface on which the film is formed, and detects intensities of regularly reflected lights from the predetermined area in 3 mutually different polarization directions (p-plane of polarization, s-plane of polarization and a direction which the principal axis tilts +45° with respect to the p- and s-plane of polarization, respectively) as the following 4 data: I1, I2, I3 and I4:
      I1    =                          Ex                    2            I2    =                          Ey                    2                                    I3          =                    ⁢                                    (                              1                /                2                            )                        ⁢                                                                            Ex                  +                  Ey                                                            2                                                                    =                    ⁢                                                    (                                  1                  /                  2                                )                            ⁢                              {                                                                                                  Ex                                                              2                                    +                                                                                  Ey                                                              2                                                  }                                      +                                                          Ex                                            ⁢                                              Ey                                            ⁢              cos              ⁢                                                          ⁢              δ                                                                        I4          =                    ⁢                                    (                              1                /                2                            )                        ⁢                                                                            iEx                  +                  Ey                                                            2                                                                    =                    ⁢                                                    (                                  1                  /                  2                                )                            ⁢                              {                                                                                                  Ex                                                              2                                    +                                                                                  Ey                                                              2                                                  }                                      +                                                          Ex                                            ⁢                                              Ey                                            ⁢              sin              ⁢                                                          ⁢              δ                                          
Where Ex represents the intensity of the s-plane polarized light component, Ey represents that of the p-plane polarized light component, δ (δ=δx−δy) represents a subtraction value of a phase difference δy between the p-plane polarized light before and after reflection from a phase difference δx between the s-plane polarized light before and after reflection. “i” represents an imaginary unit. I1 represents the intensity of the regularly reflected light from the s-plane of polarization, I2 represents that from the p-plane of polarization, and I3 represents that from a plane of polarization with a direction of its principal axis at 45° (azimuth angle), and I4 represents a intensity from a plane of polarization with the azimuth angle of 45° after light passed through a ¼ wavelength plate and transferred into a linearly polarized light.
On the other hand, 4 Stokes parameters, S0, S1, S2 and S3, which characterize polarization conditions, are defined respectively as follows:S0=|Ex|2+|Ey|2S1=|Ex|2−|Ey|2S2=2|Ex| |Ey| cos δS3=2|Ex| |Ey| sin δ
Thus, the Stokes parameters S0 to S3 can be calculated as the followings, using the 4 data of the detected light intensities I1 to I4.S0=I1+I2S1=I1−I2S2=2·I3−(I1+I2)S3=2·I4−(I1+I2)
Therefore, based on the thus obtained Stokes parameters, it is capable to obtain the optical constants (Hereinafter, these are generally referred to as physical properties of a film.), such as a film thickness d, a refractive index n and an absorption coefficient (an extinction coefficient) K and so forth. Furthermore, it is also capable to identify an unknown object which forms a coating film, in conjunction with a database wherein optical constants and corresponding chemical names are pre-defined.
However, for an ellipsometer equipped with only a single wavelength detector, since it is demanded for the ellipsometer to detect each of the 4 light intensities in the above mentioned 3 polarized light directions respectively, it is necessary to carry out detections for 4 times to obtain each of the light intensities I1, I2, I3 and I4 respectively, by sequentially switching a analyzer and a ¼ wavelength plate disposed in the optical path of a reflection light to be detected and thus the detection time is long. Therefore, it is desired to shorten the detection time.
However, in detecting a light intensity of a reflection light on a predetermined area of an inspected surface, as several iterative detections are carried out according to the numbers of the polarized light directions, the conventional ellipsometer described above has no problem in a case when a subject to be detected is only a partial area of the inspected surface; while in another case when the subject to be detected is of multiple areas, such as the overall area of an inspected surface is to be detected in detail, it is necessary for the inspected surface to be transferred manually to a next area to make another detection after a detection on one area is finished, it takes considerably long detection time and is troublesome.
As a result, there has been proposed an ellipsometer, which uses a beam splitter to split the optical path of a reflection light into 4 optical paths, in 3 of the 4 optical paths, 3 analyzers with mutually different principal axis directions are disposed accordingly, a ¼ wavelength plate and a analyzer are disposed in the left one. Based on the fact that the reflection lights passed through the analyzers in each optical path are detected respectively by the respective 4 light intensity detectors, it is possible to detect simultaneously the 4 light intensities (I1, I2, I3 and I4) of the reflection lights with different polarization conditions in one-time detection (for reference, see Japanese Patent Laid-Open H7-159131).
According to the above technology, it is capable to detect simultaneously the light intensities with 4 different polarization conditions. As no iterative detection is necessary, it is possible to shorten the detection time.
Although the technology to obtain simultaneously 4 detection results in one determination process for an area by splitting the optical path of a reflection light was described in the reference patent, it has no problem in a case when the inspected surface is a flat and smooth mirror one; while in another case when the inspected surface includes coarse surfaces rather than a mirror surface, scattered lights which are a noise component reflected on the coarse surfaces overlapped with the regularly reflected lights which are a signal component will be detected, S/N becomes lower and so does the reliability of the detection result.